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Research Article Open Access

Corneal Evaluation in Healthy Brazilian Children Using a Scheimpflug Topography System

Abstract

Purpose: To identify the distribution and variation in corneal topography, thickness, and elevation in healthy Brazilian children as measured by the Pentacam Scheimpflug system (Oculus Optikgeräte GmbH, Wetzlar, Germany).Methods: Healthy children between 7 and 11 years of age were scanned using the Pentacam Scheimpflug corneal topography system (Oculus Optikgeräte GmbH, Wetzlar, Germany). The exclusion criteria were inability to undergo the ocular exam, history of any ocular diseases (including strabismus, amblyopia, cataracts, retinal disorders, and allergic conjunctivitis), and topographic diagnosis of corneal ectasia based on the modified Rabinowitz/McDonnell criteria for keratoconus. The right eye of each subject was selected for analysis. The parameters evaluated were central corneal thickness (CCT), thinnest pachymetry (TP), average pachymetric progression index (PPIave), anterior and posterior elevation (AE and PE), anterior and posterior best fit sphere (ABFS and PBFS), pachymetric difference between the apex and the thinnest point (PDAT), Ambrosio's relational thickness (ARTMax), overall Belin/Ambrósio Enhanced Ectasia display score (BAD-D), simulated keratometry (SimK), astigmatism in SimK (SimK astig), maximum keratometry (K max), asphericity (Q value), and anterior chamber depth (ACD).Results: A total of 160 children (69 male, 91 female) were included in this study. The mean age of the children was 8.82 ± 1.23 years (ranging from 7 to 11 years). The mean CCT was 553.81 ± 32 μm, and mean TP was 547.95 ± 32.06 μm. The TP was most commonly located in the inferotemporal quadrant in 93.125% (149) eyes. The mean PPIave was 1.00 ± 0.14, similar to that of normal adults. The mean ABFS and PBFS values were 7.49 ± 3.26 and 10.54 ± 6.25, respectively. ART Max and D averaged 446.57 ± 81.20 and 0.78 ± 0.65, respectively. Mean ± SD values for SimK, SimK astig, and K max were 43.35 ± 1.31 D, 0.92 ± 0.66 D, and 44.40 ± 1.45 D respectively. The K max and SimK astig values were close to those reported by other topographic systems for children. Q value and ACD averaged -0.39 ± 0.12 and 3.065 ± 0.2745 mm.Conclusion: This study provides normative values for corneal topography, thickness, and elevation in healthy Brazilian children. These results may provide helpful information for diagnosis of corneal diseases in children. Further studies are needed to evaluate the role of tomography in identifying early forms of ectasia in this age group.

Keywords

Cornea; Children; Topography; Healthy; Pentacam;
Scheimpflug

Introduction

In clinical practice and research involving the use of keratometric
readings of the anterior surface of the eye, the study of the posterior
aspects of the cornea and its pachymetric distribution are very
important for understanding how the cornea behaves structurally.
These parameters are needed to identify abnormal corneas and to
predict how they will behave in the future and after surgical
procedures. Until techniques such as slit-scanning and Scheimpflug imaging were developed, the field of corneal imaging was restricted to
the analysis of the shape and optical quality of the cornea's anterior
surface with Placido disc-based topography systems [1]. These are still
the most commonly used technologies for cornea imaging, though
they lack the ability to diagnose subclinical keratoconus, contact lens
corneal warpage, and corneas that are not suitable for refractive surgeries. These new anterior segment imaging technologies are
capable of reconstructing the three-dimensional structure of the
cornea from two-dimensional optical cross-sections, greatly enhancing
our ability to investigate the properties of the cornea. There are
currently five devices on the market that are based on these
technologies: Orbscan IIz (Bausch & Lomb Surgical, Inc), Pentacam
(Oculus, Inc), the Galilei (Ziemer Ophthalmic Systems AG,) the
Precisio (Ligi Tecnologie Medicali), and the Sirius (CSO Ophthalmic).
The Orbscan IIz is the only slit-scanning-based technology; the others
involve Scheimpflug-based technology [2].

The Pentacam corneal tomographer (Oculus Optikgerδte GmbH,
Wetzlar, Germany) is a Scheimplug-based topographer that can
provide readings and analyses of the anterior and posterior surfaces of
the cornea, as well as measurements of corneal thickness. Since its
launch in 2004, it has become a popular device for evaluating the
anterior segment of the eye. Previous studies have demonstrated
excellent reproducibility of the automated measurements of the anterior segment structures [3-6]. The Pentacam system may be used
to diagnose keratoconus, to screen corneas for refractive surgery, to
monitor post-surgical corneas, to calculate the keratometric index and
intraocular lens power, and to assess intraocular lens implants. Studies
have provided normative data on the anterior segments of adults [7], as
well as on changes in posterior corneal elevation following the use of
an excimer laser [8]. In order to avoid undetectable keratoconus, new-generation
devices now offer features and algorithms that help to
differentiate keratoconus from normal corneas [9,10]. It is important
to recognize the influences of aging on the shape of the anterior and
posterior surfaces of the cornea [11], especially in patients who have
cataracts and who have undergone corneal refractive surgery-in other
words, patients who are in most need for accurate keratometric
parameters in the selection of the correct intraocular lens [12].

Despite the vast amount of data on these corneal parameters in
adults, similar information on children is still scarce [13], which can
limit the use of the Pentacam for identifying abnormal corneas. Some
studies have shown that the onset of keratoconus typically occurs
during puberty [14]. However, when this condition develops sooner (as
in cases of pediatric keratoconus), it seems to progress faster and to be
more advanced at the time of diagnosis than keratoconus in adults
[15]. Due to its advanced stage at diagnosis, pediatric keratoconus
bears a higher risk of corneal scarring than adult cases, an issue which
reflects a greater need for penetrating keratoplasty [16]. All of this
information justifies the importance of obtaining keratometric,
pachymetric, and elevation data from pediatric patients in order to
diagnose corneal abnormalities earlier and, ideally, to avoid
progression to such advanced stages and subsequent losses in quality of
life.

The purpose of this study was to establish a database of normative
corneal topography measurements from healthy children between 7
and 11 years of age using the Pentacam corneal topography system.

Patients and Methods

The present study was reviewed and approved by the Research
Ethics Committee of the University of Campinas (UNICAMP) in
Campinas, São Paulo, Brazil under CAAE Registry number
54921916.9.0000.5404, and the tenets of the Declaration of Helsinki
were followed. The study was based on a retrospective review of the ophthalmological medical records of healthy children enrolled in
second to fifth grade of the State of São Paulo public school system in
2011. As it was a retrospective review of the ophthalmological medical
records of patients that were examined in the past, the informed
consent was dispensed with no harm to the patients. The Honorato
Faustino State School was chosen randomly for these exams as part of
a partnership with the state government, and children from 7 to 11
years of age who agreed to participate of the study and whose parents
and guardians agreed to their participation were included. The
exclusion criteria were inability to undergo the ocular exam, history of
any ocular diseases (including strabismus, amblyopia, cataracts, retinal
disorders, and allergic conjunctivitis), and topographic diagnosis of
corneal ectasia based on the modified Rabinowitz/McDonnell criteria
for keratoconus [17]. The ocular tomographic exam was performed
using the OCULUS-Pentacam Scheimpflug Topography System. During the examinations, the patients were comfortably positioned at
the instrument with proper placement on the chin rest and forehead
strap. The patients were asked to blink a few times and to open both
eyes and stare at the fixation target. The system, which was in
automatic release mode, started the scan after proper alignment was
obtained. Twenty-five single Scheimpflug images within captured for
each eye within 2 seconds. Two experienced ophthalmologists and
corneal specialists in a dimly lit room performed all of the
tomographic exams, and the tomography was repeated until acceptable
image quality was obtained (image quality defined as “OK” according
to the built-in Examination Quality Specifications). Details are shown
in Figure 1. The analysis of the captured images considered the
topographic, pachymetric, and elevation parameters, and maps,
graphs, and indices have been created.

Figure 1: Overview screen on the Pentacam showing the camera images, the optical density of the ocular tissues, the keratometric,
pachymetric, and volumetric data of the anterior segment of the eye, and the reliability of the images as highlighted by the red circle.

Central corneal thickness (CCT) was recorded at the apex of the
cornea when the patient fixated on the target of the topographer. The
thinnest point (TP) and its distance to the CCT in micrometers (μm)
were recorded. All of these calculations are exemplified in Figure 2.

Figure 2: Corneal thickness map showing the anatomical relations
of the pupil edge (red arrow) and its center (black arrow); the
thinnest point and the center of the map represent the corneal apex.

Anterior and posterior maps were used in the calculations of the
anterior and posterior elevations at the thinnest point (AETP and
PETP), as well as anterior and posterior elevation in the best fit sphere
(ABFS and PBFS) using a fixed optical zone of 8.0 mm, as seen in Figure 3.

Figure 3: Posterior elevation map and the values based on the best
fit sphere; note the anatomical relations between the pupil edge and
its center, the thinnest point and the center of the map representing
the corneal apex.

The mean value of the pachymetric progression index (PPIave) was
recorded, as were the anterior elevation (AE) and posterior elevation
(PE). The relational thickness parameter was calculated as the ratio of
the PPI value to Ambrosio’s relational thickness (ART), which refers to
the relational thickness of the TP with PPIMax (ARTMax). Using the
Belin/Ambrosio Enhanced Ectasia display, a calculation was made
based on the anterior and posterior elevations and on the distribution
of thickness and the overall value, which was referred to as BAD-D.
The BAD-D was recorded and used in the description of how the
thickness of the cornea changed between the thinnest area and the
periphery. Another parameter recorded was the asphericity of the
cornea, which is measured by a Q value in which an oblate cornea has
a positive value and a prolate cornea has a negative value. To avoid
redundancy of the data, we included only the right eye. The data was
manually transferred to Microsoft Excel (Microsoft, Redmond,
Washington).

Statistical analyses were performed using the R Project software,
version 3.0 (R Foundation for Statistical Computing Platform, Vienna,
Austria). The mean, standard deviation, range, and confidence interval
(95% CI) were calculated for each parameter in question. The
probability of distribution was calculated using the skewness and
kurtosis descriptors.

Results

A total of 160 healthy children from 7 to 10 years of age were
included in this study. Of these subjects, 69 were male and 91 were
female (43.1% and 56.9%, respectively). The average patient age was
8.82 ± 1.23 years. Table 1 summarizes the results.

In the analysis of corneal thickness, the mean CCT was found to be
553.81 ± 32 μm (range: 481 to 640), and mean TP was 547.95 ± 32.06
μm (range: 469 to 629). The TP was most commonly located in the
inferotemporal quadrant in 93.125% (149) eyes. The distribution of
thickness was represented in PPIave and ARTMax, which were 1.00 ±
0.14 (range: 0.64 to 1.45) and 446.57 ± 81.20 (range: 223 to 745),
respectively.

The overall Belin/Ambrosio’s Enhanced Ectasia display score (BADD)
was calculated using elevation, thickness, and the distribution of
the aforementioned thickness data, and the result was 0.78 ± 0.65
(range: 1.14 to 2.42). The shape of the cornea was analyzed based on its
asphericity, which was a Q value of -0.39 ± 0.12 (range: -0.77 to -0.05).

Discussion

Before the advent of Scheimpflug technology, Placido disk-based
computer assisted videokeratoscopy was the best way to clinically
evaluate corneal topography. It detected the curvature and corneal
power of the anterior surface of the cornea, and its application is well
known, including among pediatric patients. Before Scheimpflug
technology, most classification criteria for keratoconus were based on anterior corneal curvature data derived from corneal topography
[18,19]. With the introduction of corneal tomographers such as the
Orbscan IIz (Bausch & Lomb Surgical, Inc), the Pentacam (Oculus,
Inc), and the Galilei (Ziemer Ophthalmic Systems AG,), both the
anterior and posterior corneal surfaces came to be measured along
with the pachymetry at each point, and elevation maps are now
produced. These advancements provide information that was
previously lacking with the Placido disk-based keratoscopy, and this
information helps in the diagnosis of diseases by identifying different
patterns of anterior and posterior topographies. This new data also aids
in disease monitoring by providing additional parameters and more
detailed information. These Pentacam measurements are currently
being used in the formulation of new algorithms for the diagnosis of
keratoconus and in the creation of built-in software that is capable of
categorizing corneas as normal or as having cases of subclinical
keratoconus [20]. An example of this is the new software adaptation to
the Pentacam Scheimpflug Tomography (Oculus, Wetzlar, Germany)
called the Belin/Ambrosio Enhanced Ectasia Display (BAD). The BAD
software combines anterior and posterior elevation data with
pachymetric data to provide a three-dimensional tomographic
representation of the cornea’s shape. A previous study showed that the
thickness profile provided by the Pentacam and the BAD software can
detect early keratoconus with a sensitivity and specificity of 98% [9].
The details provided by the Pentacam are reliable because the
equipment has a high level of reliability and repeatability for
keratometric and pachymetric readings [21,22]. The measurement of
corneal thickness is essential in the diagnosis and evaluation of
multiple ocular and corneal diseases, for establishing surgical
indication, and for monitoring certain pathologies. Corneal thickness
is considered an important indicator of corneal health, and it can be
evaluated through the use of several methods, including ultrasonic
pachymetry, optical slit-lamp pachymetry, confocal microscopy,
specular microscopy, and partial coherence interferometry. Several
reports have found corneal thickness measurements performed by the
Pentacam and its pachymetric map to be reliable and reproducible
[5,23-26].

With reports that the onset of keratoconus typically occurs during
puberty [14] and that vision-related quality of life is worse among
these patients [27], the detection of abnormal corneas in children has
gained significant importance. It may be difficult to distinguish
subclinical keratoconus from regular myopic astigmatism during a
routine eye exam. Proper and prompt diagnosis of keratoconus is
crucial for a better understanding of disease progression, for early
treatment, and to avoid decreases in young patients’ BCVA and the
subsequent impact on productivity and quality of life [28]. While
advanced keratoconus is easily diagnosed by slit-lamp biomicroscopy
and corneal curvature readings, diagnosing subclinical keratoconus
remains a challenge. Another benefit of topography evaluation is
greater sensitivity to the subtle changes in topographic parameters in
children with subclinical expressions of keratoconus and suspicious
topographic corneal patterns. In patients who have already been
diagnosed with keratoconus, topography exams are important for
analyzing progression and establishing surgical indication.

Previous studies have provided substantial information and many
normative values for corneal power, astigmatism, corneal thickness
and pachymetric progression indices in adults. However, these
parameters have not been studied in children using current
tomographers such as the OCULUS-Pentacam Scheimpflug
topography system. Our study provides normative data resulting from
the use of Pentacam Scheimpflug corneal topography in children from 7 to 11 years of age. This data may be useful for distinguishing between
normal and abnormal corneas. It has the potential to aid in the early
diagnosis of corneal abnormalities, in the evaluation and monitoring
corneal ectasia, and in the understanding of changes in corneal
curvatures in healthy patients, in cases of ocular diseases, and in
children who have undergone eye surgeries. Along with other
topographic and tomographic systems, the Pentacam is one of the best
technologies for scanning corneas in children. It provides data on
topography, corneal thickness, and elevations with a single, rapid scan.
In this study, topographic and pachymetric parameters of healthy
children’s corneas were studied using the Pentacam Scheimpflug
topography system, as well as pachymetric progression indices and
Ambrosio’s relational thickness. As stated previously, precise
knowledge on corneal topography in children is important for corneal
ectasia diagnoses, contact lens fitting, and ocular surgery assessments
[29].

This study on healthy children between 7 and 11 years of age found
a relative similarity between children’s mean CCT, TP, PPIave and
BAD-D values and those of normal adults. The results also reflected a
relative difference between children’s values and the values in
subclinical and keratoconus patients found by Ambrósio et al. [9], that
studied 113 normal and 44 eyes with keratoconus from brazillian
patients aged between 11.7 to 78 years (mean 38.7 ± 17.9 years), and
Vazquez et al. [30], that studied 189 normal and 44 eyes with
keratoconus and their fellow eyes from argentine patients aged
between 14 and 71 years (mean 32.3 ± 8.1 years), as seen in Table 2. It
seems that children’s corneas become thinner from the periphery to
the center the same way and as fast as adult corneas do, a finding that
suggests that this parameter could be used to identify pediatric corneas
that differ from healthy parameters. More studies are needed to
significantly distinguish between normal and abnormal corneas based
on these parameters.

Table 2: Measurements of 160 children’s eyes and cutoff values in the current study and as reported in other studies.

K max (42.95 ± 1.32) and SimK astig (0.92 ± 0.66) values were
found to be similar to those found through the use of other
topographic systems with older technologies (computerized
videokeratoscopy) and newer technologies (slit-scanning and Scheimpflug) among healthy children and adults [29,31]. These values
represent a low overall level of corneal astigmatism in the pediatric
population of this study. These indices were also found to be similar to
those of other studies (Table 3).

Method

No. of eyes

SimK max (D)

SimK min (D)

SimK Astig (D)

Current Study

Pentacam

160

43,86

42,95

0,92

Ortiz-Toquero et al.

Galilei G4

56

44,22

43,22

0,97

Ortiz-Toquero et al.

Allegro-Topolyzer

56

44,07

43,12

0,95

Reddy et al.

Orbscan II z

100

44,26

43,56

0,69

Liu et al.

Orbscan

94

44,24

43,31

0,9

Bogan et al.

CMS

216

43,39

0,8

Rabinowitz et al.

TMS-1

390

43,7

Table 3: Simulated keratometry (SimK) findings in different studies.

As Table 4 shows, we also compared the parameters between males
and females and found the values to be similar. The mean Q value was
-0.39 ± 0.12, a finding which suggests that children’s corneas are more
prolate than those of African-American adults (-0.26 ± 0.19) and of Caucasian adults (-0.20 ± 0.12) in studies that used the Pentacam [32].
Younger corneas seem to be more prolate than those of older patients
(7 years -0,3919 ± 0,02438; 11 years -0,3309 ± 0,03195; p=0.1428), but further studies on more subjects are needed to prove this information
with statistical significance.

This study is the first to report on the corneal parameters of healthy
Brazilian children. Some limitations must be noted, however: though
the parameters are similar to those found in the general population,
the age range of our study was between 7 and 11 years, and it is not
known how the corneas of younger and older children behaves and
whether these same parameters can be applied to other pediatric age
groups. Because the current study is descriptive in nature, its statistical
significance cannot be compared to that of other populations (such as
adults and patients with keratoconus corneas). Therefore, it is difficult
to determine similarities and differences between these findings and
others. It is also important to note that, while the tomographic exam
requires only two seconds capturing all of the images, the patient to
remain immobile and fixate on the target, which could be difficult to
achieve among younger patients.

This is the first study in the literature to provide normalized values
for keratometry, pachymetry, and relational data from Brazilian
children’s corneas using the Pentacam Scheimpflug topography system.
It can provide a good foundation for the comparison of abnormal pediatric corneal topography to normal cases. This data may prove
useful in the field to help distinguish between normal and abnormal
corneas, as well as in future comparative studies that consider different
corneal diseases in children. Further studies are needed to evaluate the
role of tomography in identifying early forms of ectasia in children.